In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
One type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
Since the late 1950s and early 1960s, a new type of geophysical prospecting, generally known as “VIBROSEIS”® prospecting, has been used. Vibroseis prospecting employs a land or marine seismic vibrator as the energy source. In contrast to an impulsive energy source, a seismic vibrator imparts a signal into the earth having a much lower energy level, but for a considerably longer period of time.
The seismic signal generated by a seismic vibrator is a controlled wavetrain (i.e., a sweep) which is applied to the surface of the earth or in the body of water or in a borehole. In seismic surveying on land using a vibrator, to impart energy into the ground in a swept frequency signal, the energy is typically imparted by using a hydraulic drive system to vibrate a large weight (the reaction mass) up and down. The reaction mass is coupled to a baseplate, in contact with the earth and through which the vibrations are transmitted to the earth. The baseplate also supports a large fixed weight, known as the hold-down weight. Typically, a sweep is a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range, which is applied during a sweep period lasting from 2 to 20 seconds or even pore. The frequency may vary linearly or nonlinearly with time. Also, the frequency may begin low and increase with time in a so-called upsweep, or it may begin high and gradually decrease in a downsweep.
Vibrators for use in marine seismic surveying typically comprise a bell-shaped housing having a large and heavy diaphragm, equivalent to the aforementioned baseplate, in its open end. The vibrator is lowered into the water from a marine survey vessel, and the diaphragm is vibrated by a hydraulic drive system similar to that used in a land vibrator. Alternative marine vibrator designs comprise two solid curved or hemispherical shells, joined together by an elastic membrane. The hydraulic drive moves the two shells relative to one another in a similar manner to the movement of the reaction mass in a land vibrator.
The seismic data recorded during Vibroseis prospecting (hereinafter referred to as “vibrator data”) are composite signals, each consisting of many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. However, when the seismic vibrator data is cross-correlated with the sweep signal (also known as the “reference signal”), the resulting correlated data approximates the data that Would have been recorded if the source had been an impulsive energy source.
The amount of energy injected into the earth during a conventional vibrator sweep is governed by the size of the vibrator and the duration of the sweep. Emerging and beyond imaging applications of the seismic method such as acoustic impedance inversion and in general reservoir characterization and monitoring require signals with energetic low frequencies. On the receiver side, the growing use of accelerometers in surface and borehole seismic enables the acquisition at high signal to noise ratio of frequencies lower than 8-10 Hz. Low actuator forces are needed to shake the vibrators at these low frequencies (typically 4 to 8 Hz), which in turn can yield extremely long sweeps in absence of a design criterion or if a too conservative criterion is used.
There are several of constraints on the amplitude of the vibrations. The most basic of these is that the hold-down weight must exceed the maximum upward force, so that the vibrator never loses contact with the ground. However, there are other constraints on low frequency output. Since, as already mentioned, the ground force is generated by vibrating a large weight, and the force generated by the weight is equal to its mass times its acceleration, at low frequencies for the same generated ground force the peak velocities and displacements are higher than at high frequencies. Typically, the lowest frequency that can be produced by a vibrator at a fixed force level is determined by the maximum stroke length possible for the vibratory weight, and the amount of time that the vibrator can spend at low frequencies is determined by the amount of hydraulic fluid stored in accumulators at the start of the sweep time and the maximum flow capacity of the hydraulic system. Marine vibrators are therefore subject to operational constraints analogous to those of land vibrators.
U.S. Pat. No. 5,410,517 issued to Andersen discloses a method for cascading or linking vibrator sweeps together to form a cascaded sweep sequence and optionally eliminating the listen period between successive sweeps. The initial phase angle of each individual sweep segment within a sweep sequence is progressively rotated by a constant phase increment of about 360/N degrees, where N is the number of sweep segments within the sweep sequence. Either the correlation reference sequence or the vibrator sweep sequence, but not both, contains an additional sweep segment positioned and phased so as to substantially suppress harmonic ghosts during correlation. When the additional sweep segment is included at the end of the vibrator sweep sequence, it increases the total acquisition time. If the correlation reference sequence includes the additional sweep segment, it complicates the processing in that the additional sweep segment has to be input at negative time giving a nonstandard correlation operator.
In the U.S. Pat. No. 7,050,356 issued to Jeff ryes there is disclosed a method of seismic acquisition using multiple vibrators using the so-called “slip-sweep” method. The method consists of a vibrator (or a vibrator group) sweeping without waiting for the previous vibrator's sweep to terminate. Correlation, which acts as a time-frequency filter, then extracts the individual records. A significant reduction in overall acquisition time is obtained. This is more efficient than the cascaded sweep since there is no need to wait for the end of a sweep before starting the next sweep. The reduction in overall acquisition time comes at the cost of increased harmonic distortion since the harmonics from the second sweep will contaminate the primary signals of the first sweep. The patent also describes methods to estimate and remove the harmonics.
U.S. Pat. No. 6,418,079 issued to Fleure discloses a method for segmenting the spectral distribution of overlapped vibratory signals, thereby improving the efficiency of data acquisition while providing reduced harmonic distortion in the time zones of interest. Two identical sweep segments are used. Each sweep segment includes an earlier low frequency sweep and a later high frequency sweep, the individual sweeps having substantially no overlap in frequency except for tapering. The high frequency sweep in each pair starts before the end of the low frequency sweep with an overlap in time that is selected to avoid harmonics from the low frequency sweep. Correlation of the recorded signal separately with the low frequency sweep and the high frequency sweep gives data sets in which individual portions of the desired data are recoverable with the harmonic distortion largely separated from the desired data.
Another prior art way of seeking to overcome the problems resulting from the various constraints on land or marine vibrator operation is disclosed in U.S. Pat. No. 6,181,646 issued to Bouyoucos and Hollinger. The vibrator source of the system (hereinafter referred to as the prior art system) described in that patent is driven so as to provide a composite sweep, in which a high frequency sweep and a low frequency sweep are carried out concurrently over the same time interval, i.e., both sweeps start at the same time and finish at the same time.
In the UK patent application GB-2416033-A of Jeffryes and Martin, a method is described to provide a composite sweep with a low and high frequency part.
Another method to provide a composite sweep for marine vibrators is described in the U.S. Pat. No. 6,942,059 B2 issued to Smith. The desired vibroseis bandwidth is apportioned over a plurality of vibroseis projectors (sources).
E. Rietsch describes in: “Vibroseis signals with prescribed power spectrum”, Geophysical prospecting, vol 25, 1977, pages 613-620, basic methods to control vibrators for generating a desired power spectrum.
In the published US patent application 2007/0133354 A1 of Bagaini there is disclosed a method, herein referred to as maximum displacement sweep design, to enhance the low frequency content of Vibroseis acquisitions using sweeps designed to optimally use the vibrator's mechanical specifications.
In view of the work on composite sweeps of high and low frequency part, it is observed that the composite signal often contains artifacts which can distort the observed vibroseis data. The present invention therefore seeks to provide novel methods of generating a composite sweep using two or more concurrently operated vibrators.